Lignite

Lignite is a low-rank coal (essentially synonymous with "brown coal"), and low-rank coals have been recognized as being more suitable than other types of coal for production of organic agricultural products.
Naturally oxidized lignite has been referred to as "leonardite". This form of lignite is probably the richest in readily available humic substances and has been the most widely used raw material for extraction of humic acid by alkali solutions.
Leonardite is naturally oxidized lignite, and natural oxidation has enhanced the humic acid content above the content in un-oxidized lignite.
Lignite is recognized as a highly valuable soil conditioner for improving soil properties and enhancing crop yields. During the process of formation, microorganisms seemed to have played an important role in the chemical configuration of lignite (peat being the precursor) at least during the early years of burial of biomass. Both aerobic and anaerobic microbes are thought to be involved in the process. Gelification of biomass (mostly of gymnosperms, although contribution of angiosperms has also been suggested based on the evidence gathered through state-of-the-art analytical methodologies) would appear to be mediated mainly by aerobic microbes. The particular configuration of lignite that entails preservation of oxygen-containing functional groups like OH and COOH gives this poor quality coal a unique feature of physiological functionality when treated with hydroxide of ammonium, potassium or sodium. The resulting water soluble entities i.e. Humates (humic acid and fulvic acid) have now been unequivocally established as having an immense impact on physiological functioning of plants and animals alike. Humates released from lignite in soluble forms have multiple benefits when applied to soil and plants or fed to animals as food supplement.